1. Field of the Invention
This invention generally relates to a developing device for use in developing a latent image in an image processing machine, such as a copier, printer and facsimile machine, and in particular to a developer carrier for use in such a developing device and process for manufacturing such a developer carrier. More specifically, the present invention relates to an elastic developer carrier to be incorporated in a developing device for carrying thereon a film of toner to be applied to an electrostatic latent image formed on an imaging member, such as a photosensitive member, for development of the same and a process for manufacturing such an elastic developer carrier.
2. Description of the Prior Art
In an electrostatic recording apparatus, such as an electrophotographic copier, printer and facsimile machine, different developing characteristics are required depending on the kind of an original as to whether the original mainly consists of a line image or an area image. FIG. 1 is a graph showing ideal developing characteristics in which the abscissa is taken for the density of original image and the ordinate is taken for the density of copy image. In the graph of FIG. 1, the solid curve A indicates an ideal characteristic for an original mainly consisting of an area image; on the other hand, the dotted curve B indicates an ideal characteristic for an original mainly consisting of a line image. As seen easily, the curve B for a line image original is steeper in rising slope as compared with the curve A for an area image original. This is based on the fact that in the case of an original consisting of a line image, it is normally required to increase the image density of copied image even if the image density of original image is relatively lower. On the other hand, in the case of an original consisting of an area image, it is normally required that the image density of copied image is substantially proportional to the image density of original image.
In increasing the image density of copy image in the case of a line image original, the so-called edge effect has been typically used. The edge effect is a phenomenon in which the electric field strength is locally increased at the periphery of an electrostatic latent image as compared with the central portion thereof so that more toner may be attracted to the peripheral portion of the latent image. Thus, in the case of a line image original, since almost all of the region defining the image corresponds to the peripheral portion thereby receiving an increased amount of toner so that the image density may be increased when developed. Such an edge effect has been sufficiently attained in a system using a conventional two-component developer including toner and iron powder. However, in a system using a single-component toner, or magnetic toner, there has been difficulty in obtaining such an edge effect to a sufficient degree.
Under the circumstances, there has been proposed a novel developer carrier allowing to obtain such an edge effect sufficiently even using a single-component developer as disclosed in Japanese Patent Application No. 55-185726, which has been assigned to the assignee of this application. As shown in FIG. 2, the developer carrier proposed in the above-mentioned application includes an electrically conductive, cylindrical support 1 and an electrode layer 2 formed on the support 1 such that a plurality of semi-spherical electrode particles 2a are provided as electrically isolated from one another as partly embedded in a dielectric material as partly exposed at an outer peripheral surface. When incorporated into a developing device, a magnet roll 3 is fitted into the cylindrical support 1 so as to form a magnetic force line at the outer peripheral surface of the developer carrier thereby causing magnetic toner to be magnetically attracted to the outer peripheral surface. Typically, the magnet roll 3 is so supported that it can be driven to rotate independently of the developer carrier.
Such a structure allows to obtain the ideal developing characteristics depending on the kind of originals to be copied as shown in FIG. 1. However, such a structure also suffers from other disadvantages. That is, in order to obtain a sufficient magnetic force at the outer peripheral surface of the developer carrier, the magnet roll 3 tends to become larger in size, which, in turn, causes the entire developing device to be rather bulky. Besides, a gap G between the magnet roll 3 and the inner surface of the cylindrical support 1 must be properly adjusted and maintained so as to form a film of toner uniform in thickness on the outer peripheral surface of the developer carrier. Uniformity in such a film of toner is important in maintaining uniformity in developing performance across the entire outer peripheral surface of the developer carrier. It will be easily understood that a narrow tolerance for gap G presents difficulty in assembling the present developer carrier into a developing device, which could also push up the cost. Besides, the embedded depth t.sub.2A of each of electrode particles 2a is required to fall on a certain range in order to attain the desired edge effect, which cannot be realized without special considerations.